The present invention relates generally to the field of measuring the size distribution of particles and more particularly to a method and apparatus for measuring the size distribution of very small particles.
In the past, a number of methods have existed for determining the size distribution of particulate material for particles in the approximate size range of 0.1 to 100 microns in diameter. For
U.S. Pat. No. 3,873,206 to William Leslie Wilcock, example, issued Mar. 25, 1975, and U.S. Pat. No. 4,134,679 to Allen L. Wertheimer, issued Jan. 16, 1979, both assigned to the assignee of the present invention, describe such methods. In addition dynamic scattering instruments are capable of determining the size of particles by measuring their Brownian motion. Brownian motion is caused by random collisions between the particle and thermally excited molecules of the dispersing media. The velocity and direction of the motion is random, however the velocity distribution of many particles averaged over a long time will approach a known functional form. Since small particles are known to move faster than larger particles, the particle size can be determined by measuring this size dependent velocity distribution. For example, fiber optic doppler anemometers such as those disclosed in U.S. Pat. No. 4,637,716, to Auweter et al, patented Jan. 20, 1987, and U.S. Pat. No. 4,818,071 to Dyott, patented Apr. 4, 1989, are capable of measuring the size of very small particles down to a diameter of approximately 0.005 microns in diameter. However, such fiber optic doppler anemometers have been useful for measuring particle size accurately only when all particles are of a uniform size. Heretofore, there has been no known method of accurately measuring the particle size and distribution of very small particles of multiple sizes.